Digital printing techniques have been developed that allow a printer to receive instructions directly from a computer without the need to prepare printing plates. Amongst these are color laser printers that use the xerographic process. Color laser printers using dry toners are suitable for certain applications, but they do not produce images of a photographic quality acceptable for publications, such as magazines.
A process that is better suited for short run high quality digital printing is used in the HP-Indigo printer. In this process, an electrostatic image is produced on an electrically charged image bearing cylinder by exposure to laser light. The electrostatic charge attracts oil-based inks to form a color ink image on the image bearing cylinder. The ink image is then transferred by way of a blanket cylinder onto paper or any other printing medium, the substrate.
Inkjet and bubble jet processes are commonly used in home and office printers. In these processes droplets of ink are sprayed onto a final substrate in an image pattern. In general, the resolution of such processes is limited due to wicking by the inks into paper substrates. Fibrous substrates, such as paper, generally require specific coatings engineered to absorb the liquid ink in a controlled fashion or to prevent its penetration below the surface of the substrate. Using specially coated substrates is, however, a costly option that is unsuitable for certain printing applications. Furthermore, the use of coated substrates creates its own problems in that the surface of the substrate remains wet and additional costly and time consuming steps are needed to dry the ink so that it is not later smeared as the substrate is being handled, for example stacked or wound into a roll. Furthermore, excessive wetting of the substrate causes cockling and makes printing on both sides of the substrate (also termed perfecting or duplex printing) difficult, if not impossible. Additionally, direct inkjet printing may result in poor image quality because of variation of the distance between the print head and the surface of the substrate.
Using a printing technique based on an intermediate transfer step overcomes many problems associated with inkjet printing directly onto the substrate. It allows the distance between the surface of the image transfer member and the inkjet print head to be maintained constant, and reduces wetting of the substrate as the ink can be dried on the image transfer surface before being applied to the substrate. Consequently, the final image quality on the substrate is less affected by the physical properties of the substrate.
The use of transfer members which receive ink droplets from an ink or bubble jet apparatus to form an ink image and transfer the image to a final substrate have been reported in the patent literature. Various ones of these systems utilize inks having aqueous carriers, non-aqueous carrier liquids or solid inks that have no carrier liquid at all.
The use of aqueous based inks has a number of distinct advantages. Compared to non-aqueous based liquid inks, the carrier liquid is not toxic and there is no problem in dealing with the liquid that is evaporated as the image dries. As compared with solid inks, the amount of material that remains on the printed image can be controlled, allowing for thinner printed images and more vivid colors.
Generally, a substantial portion or even all the liquid is evaporated from the image on the transfer member before the image is transferred to the final substrate, in order to avoid bleeding of the image into the structure of the final substrate. Various methods are described in the literature for removing the liquid, including heating the image and a combination of coagulation of the image particles on the transfer member, followed by removal of the liquid by heating, air knife or other means.
Co-pending PCT application No. PCT/IB13/51716 (corresponding to the U.S. national stage application having the Ser. No. U.S. 14/382,751), which claims priority from U.S. Provisional Patent Application No. 61/606,913, (both of which application are herein incorporated by reference in their entirety), teaches a printing process designed to use aqueous inks. The disclosure of the latter application overlaps with disclosure provided herein but it should be made clear that the present invention is not restricted in its application to such a process and may be used in any printing system that uses an intermediate transfer member constructed as a flexible belt regardless of whether or not the ink is water based, hence regardless of the type of release layer suitable to accommodate the ink or printing process being used.